1. Field of the Invention
This invention relates to an engine control system for a vehicle with an automatic transmission, and more particularly to an engine control system for a vehicle with an automatic transmission which prevents torque shock arising in the gear-shifting process.
2. Description of the Prior Art
In an automatic transmission, generally gear-shifting is done automatically according to the motional state of the vehicle on the basis of a predetermined shift pattern. That is, a shift pattern in which gear speeds are set according to vehicle speed and engine load (throttle opening) is stored in a control section and the automatic transmission is controlled on the basis of the shift pattern.
As a system for controlling an engine in a vehicle with such an automatic transmission in order to prevent torque shock arising in the gear-shifting process, there has been known a fuel supply system such as that disclosed in Japanese Unexamined Patent Publication No. 56(1981)-96129. In consideration of the fact that the torque of the output shaft of the automatic transmission is temporarily increased after an upshift signal is generated to upshift, for instance, from first to second or from second to third, the fuel supply system reduces the amount of fuel to be supplied to the combustion chambers (or fuel pressure) and lowers the engine output power, thereby restraining the torque increase.
However, in some cases, the fuel supply system cannot effectively prevent the shift shock. That is, detailed inspection of torque fluctuation during upshifting reveals that when the upshift signal is generated and the shifting action actually takes place in the transmission, the output shaft torque first lowers due to an increase in frictional resistance and the like before the transmission is released from the preceding state, and then increases. (See line B1 in FIG. 3.) Accordingly, if the fuel supply is simply reduced in response to an upshift, the engine output power can be lowered at a time other than the time the output shaft torque is apt to increase, e.g., at the time the output shaft torque is lowered, producing an untimely lowering of the output shaft torque, thereby causing a shift shock.
Though it is proposed, in the Japanese unexamined patent publication referred to above, to retard the timing of the lowering of the fuel pressure, taking into account the delay of the action of the transmission when upshifting into third, the timing of the lowering of the fuel pressure can deviate from the timing of the increase in output shaft torque due to error in the setting of the time intervals at which the fuel pressure is lowered. Further, the retardation is not effected when upshifting into second. Thus, this approach cannot satisfactorily overcome the problem described above.
On the other hand, shift shock can be generated also in downshifting, for instance, as a result of a torque peak produced near the completion of the shifting action of the automatic transmission. That is, the shifting action begins to take place a certain time-lag after a downshift signal is generated, and the output shaft torque of the automatic transmission fluctuates in response to the shifting action. Since the torque value after the downshift is higher than that before the downshift, the output shaft torque increases relatively quickly as the shifting action progresses. However, near the completion of the shifting action, the output shaft torque can excessively increase to produce a peak torque before the output shaft torque converges on the torque value after the downshift. When the peak torque is high, a shock is generated. This problem may be overcome by lowering the engine output power in synchronization with the gear-shifting signal. However, this approach is disadvantageous in that acceleration performance immediately after a downshift is lowered, for instance, in the case of a downshift for acceleration purposes.
In our U.S. patent application Ser. No. 808,172 filed on Dec. 12, 1985, there is disclosed an automatic transmission control system in which completion of gear-shifting is detected on the basis of both the rate of change in rotational speed of the output shaft of the torque converter and the rotational speed at which the output shaft of the torque converter is expected to rotate. However, in this system, it is not detected that the actual shifting action of the gear mechanism is in progress.